“Orbit of Mars – Kepler”;
The model that has been described is the orbit of mars as deduced by Kepler assuming the earth stands still. Kepler obtained Tycho’s observational data and decided to use it to deduce the orbit of mars. From his diagram, he wanted to say that the orbit of mars seems extremely complicated if viewed from earth, while assuming that the earth stands still and suggests that Copernicus provides the answer to this problem by posing a more dynamic and believable system while Ptolemy would use his astronomical tools to explain the motion.
An ancient Greek, Ptolemy viewed the universe as geocentric and believed that everything, including the sun, revolved around the earth. Along with Aristotelian physics, this model of the world was the one everyone accepted until years ahead in time, Copernicus came up with a separate system that he believed was true. He suggested the sun as the center of the universe and that the earth is actually a planet that revolves around the sun. although this model is referred to as the modern design, the first person to suggest heliocentrism was actually Aristarchus (who lived before Ptolemy). This led to the birth of a cosmology different to that of Ptolemy and Aristotle’s – the ‘Copernican’ system. Copernicus was followed by Brahe who came up with an explanation in which the earth formed the center of the universe and said that the sun revolves around the earth and that everything else revolves around the sun. Kepler met Tycho Brahe and inherited all of his observations. And Kepler was a more avid follower of the Copernican system than of the Ptolemaic.
Kepler, I his early years (around 1596), wrote Mysterium Cosmographicum. It it, his views are not as defined as they were in his later years, but nonetheless are termed as his early epiphany. He looked at the universe and decided to not formulate or think about the numbers of the universe after thinking and consequently, struggling about them for a while. In the end, he said that the world has six distinct spheres in the shape of the distinct Platonic shapes- the octahedron, icosahedron, dodecahedron, tetrahedron and cube. Each shape was between two successive planets so that the octahedron was between mercury and Venus, the icosahedron between Venus and earth, the dodecahedron between earth and mars, the tetrahedron between mars and Jupiter and the cube between Jupiter and Saturn. And in the universe, moving from outer planets to inner planets, the orbital period increases its ratio as twice the difference in the orb radius. But, later he gave up on this system and moved on to work on mars and work that would eventually lead him to his three laws.
Kepler started working on the orbit of mars in A.D. 1600 and spent a huge chunk of his years and energy on struggling with the orbit of Mars. His discoveries on the orbit of mars led to his first two laws that he surmised in his Astronomia Nova. He observed mars’ orbit very keenly (more than Ptolemy) and found that the observations don’t stay in accordance with Ptolemy’s equant theory and thus, Kepler gave up on equant. He could have explained mars orbit as seen from a stationary earth by adding epicycles on deferents but he was trying to come up with a more dynamic system and thus, was not content by explaining a particular observation by using astronomical tricks. So he turned to pining down mars’ orbit and for that he needed earth’s orbit. He fixed mars’ orbit and used the sun to get the earth’s orbit. He knew that mars was at the same position after every 687.1 days (mars’ orbital time). He took that point as mars’ fixed point and used that point along with the sun to find the earth’s orbit. He found earth’s orbit to be circular as opposed to mars’ which he found to oval.
Kepler wanted a wholesome system that could explain the causation of the movement of all the planets rather than explain the motion of each planet separately. Using mars, he found that the lines joining the aphelion and perihelion of its eccentric join at the sun and that the lines do not join at some point close to the sun. Thus, he deduced from this that the causation of the motion of all the planets lies in the sun. He displaced the Copernican ‘mean sun’ and replaced it with the real sun. He stumbled upon the theory that the earth is attracted for half of the orbit and then repulsed for the next half of its orbit. Thus, Kepler placed this force as a quasi-magnetic force that is exerted by the sun on the earth, as was suggested by Gilbert. He further found that the velocity of earth increased when it was closer to sun and that earth slowed down when it was farther from sun. This lead to his second law that suggested the line joining the planets to the sun sweeps out equal areas in equal periods of time.
Since Kepler displaced the earth from the center of the universe, a problem he faced was explaining the cohesion of the earth. Kepler dealt with this problem in two ways. At first, he said that bodies do not move unless they are under the influence of another body and that heaviness of a body is actually its tendency to attract or coalesce. He related tides to his theory by saying that tides actually occurred due to the moons attractive pull on the earths water. This, he stated was the reason of earth’s cohesion. Coming back to mars, Kepler noticed that the shape of mars’ orbit is actually oval and came up with his first law that surmised that planets move in elliptical orbits with the Sun at the focus.
Kepler, however, was only one of the many figures that were responsible from the shift from the traditional view of the cosmos to the more modern view of the world. His work went to inspire Galileo to work on his theories that would lead to the culmination of the struggle between the traditional and the modern astronomical models.